Rap Genius = crowd sourcing explanations of rap lyrics (and poetry [1], and other literature). It's brilliantly useful, loved by many, and has great traction.

Kurikku = crowd sourcing explanations of computer code. Also brilliant useful, probably won't have as much traction as Rap Genius just b/c there are many more people in the world interested in lyrics than in computer code, but it will probably find a StackOverflow-level popularity, which would be quite a success.

[1]:http://poetry.rapgenius.com/